We have investigated the effects of histone variants on chromatin structure. Sea urchin sperm histones lead to major alterations in core particle organization. Less marked, but significant, effects on the nucleosome are generated by the presence of the histones which are synthesized during early embryogenesis in sea urchin. Sperm histones lead to a marked stabilization of higher order structure of chromatin without altering the basic organization of the complex. Yeast plasmids containing defined, cloned yeast genes exist as minichromosomes, packaged with histones into nucleosomes. A heterologous transcription system for yeast 5S RNA genes has been established using enzyme and factors from HeLa cells. The product is the yeast 5S precursor; addition of a yeast soluble extract processes the precursor to the mature RNA. Constructed deletion mutants demonstrate that the yeast 5S promoter recognized by the mammalian enzyme is intragenic.